RTEMS 4.10.2 On-Line Library

Chapter 31: Glossary

A term used to describe an object
which has been created by an application.

aperiodic task

A task which must execute only at
irregular intervals and has only a soft deadline.

application

In this document, software which makes
use of RTEMS.

ASR

see Asynchronous Signal Routine.

asynchronous

Not related in order or timing to
other occurrences in the system.

Asynchronous Signal Routine

Similar to a hardware
interrupt except that it is associated with a task and is run in
the context of a task. The directives provided by the signal
manager are used to service signals.

awakened

A term used to describe a task that has
been unblocked and may be scheduled to the CPU.

big endian

A data representation scheme in which
the bytes composing a numeric value are arranged such that the
most significant byte is at the lowest address.

bit-mapped

A data encoding scheme in which each bit
in a variable is used to represent something different. This
makes for compact data representation.

block

A physically contiguous area of memory.

blocked

The task state entered by a task which has
been previously started and cannot continue execution until the
reason for waiting has been satisfied.

broadcast

To simultaneously send a message to a
logical set of destinations.

BSP

see Board Support Package.

Board Support Package

A collection of device
initialization and control routines specific to a particular
type of board or collection of boards.

buffer

A fixed length block of memory allocated
from a partition.

calling convention

The processor and compiler
dependent rules which define the mechanism used to invoke
subroutines in a high-level language. These rules define the
passing of arguments, the call and return mechanism, and the
register set which must be preserved.

Central Processing Unit

This term is equivalent to
the terms processor and microprocessor.

chain

A data structure which allows for efficient
dynamic addition and removal of elements. It differs from an
array in that it is not limited to a predefined size.

coalesce

The process of merging adjacent holes into
a single larger hole. Sometimes this process is referred to as
garbage collection.

Configuration Table

A table which contains
information used to tailor RTEMS for a particular application.

context

All of the processor registers and
operating system data structures associated with a task.

context switch

Alternate term for task switch.
Taking control of the processor from one task and transferring
it to another task.

control block

A data structure used by the
executive to define and control an object.

core

When used in this manual, this term refers to
the internal executive utility functions. In the interest of
application portability, the core of the executive should not be
used directly by applications.

CPU

An acronym for Central Processing Unit.

critical section

A section of code which must be
executed indivisibly.

CRT

An acronym for Cathode Ray Tube. Normally used
in reference to the man-machine interface.

deadline

A fixed time limit by which a task must
have completed a set of actions. Beyond this point, the results
are of reduced value and may even be considered useless or
harmful.

device

A peripheral used by the application that
requires special operation software. See also device driver.

device driver

Control software for special
peripheral devices used by the application.

directives

RTEMS' provided routines that provide
support mechanisms for real-time applications.

dispatch

The act of loading a task's context onto
the CPU and transferring control of the CPU to that task.

dormant

The state entered by a task after it is
created and before it has been started.

Device Driver Table

A table which contains the
entry points for each of the configured device drivers.

dual-ported

A term used to describe memory which
can be accessed at two different addresses.

embedded

An application that is delivered as a
hidden part of a larger system. For example, the software in a
fuel-injection control system is an embedded application found
in many late-model automobiles.

envelope

A buffer provided by the MPCI layer to
RTEMS which is used to pass messages between nodes in a
multiprocessor system. It typically contains routing
information needed by the MPCI. The contents of an envelope are
referred to as a packet.

entry point

The address at which a function or task
begins to execute. In C, the entry point of a function is the
function's name.

events

A method for task communication and
synchronization. The directives provided by the event manager
are used to service events.

exception

A synonym for interrupt.

executing

The task state entered by a task after it
has been given control of the CPU.

executive

In this document, this term is used to
referred to RTEMS. Commonly, an executive is a small real-time
operating system used in embedded systems.

exported

An object known by all nodes in a
multiprocessor system. An object created with the GLOBAL
attribute will be exported.

external address

The address used to access
dual-ported memory by all the nodes in a system which do not own
the memory.

FIFO

An acronym for First In First Out.

First In First Out

A discipline for manipulating entries in a data structure.

floating point coprocessor

A component used in
computer systems to enhance performance in mathematically
intensive situations. It is typically viewed as a logical
extension of the primary processor.

freed

A resource that has been released by the
application to RTEMS.

global

An object that has been created with the
GLOBAL attribute and exported to all nodes in a multiprocessor
system.

handler

The equivalent of a manager, except that it
is internal to RTEMS and forms part of the core. A handler is a
collection of routines which provide a related set of functions.
For example, there is a handler used by RTEMS to manage all
objects.

hard real-time system

A real-time system in which a
missed deadline causes the worked performed to have no value or
to result in a catastrophic effect on the integrity of the
system.

heap

A data structure used to dynamically allocate
and deallocate variable sized blocks of memory.

heterogeneous

A multiprocessor computer system composed of dissimilar processors.

homogeneous

A multiprocessor computer system composed of a single type of processor.

ID

An RTEMS assigned identification tag used to
access an active object.

IDLE task

A special low priority task which assumes
control of the CPU when no other task is able to execute.

interface

A specification of the methodology used
to connect multiple independent subsystems.

internal address

The address used to access
dual-ported memory by the node which owns the memory.

interrupt

A hardware facility that causes the CPU
to suspend execution, save its status, and transfer control to a
specific location.

interrupt level

A mask used to by the CPU to
determine which pending interrupts should be serviced. If a
pending interrupt is below the current interrupt level, then the
CPU does not recognize that interrupt.

Interrupt Service Routine

An ISR is invoked by the
CPU to process a pending interrupt.

I/O

An acronym for Input/Output.

ISR

An acronym for Interrupt Service Routine.

kernel

In this document, this term is used as a
synonym for executive.

list

A data structure which allows for dynamic
addition and removal of entries. It is not statically limited
to a particular size.

little endian

A data representation scheme in which
the bytes composing a numeric value are arranged such that the
least significant byte is at the lowest address.

local

An object which was created with the LOCAL
attribute and is accessible only on the node it was created and
resides upon. In a single processor configuration, all objects
are local.

local operation

The manipulation of an object which
resides on the same node as the calling task.

logical address

An address used by an application.
In a system without memory management, logical addresses will
equal physical addresses.

loosely-coupled

A multiprocessor configuration
where shared memory is not used for communication.

major number

The index of a device driver in the
Device Driver Table.

manager

A group of related RTEMS' directives which
provide access and control over resources.

memory pool

Used interchangeably with heap.

message

A sixteen byte entity used to communicate
between tasks. Messages are sent to message queues and stored
in message buffers.

message buffer

A block of memory used to store
messages.

message queue

An RTEMS object used to synchronize
and communicate between tasks by transporting messages between
sending and receiving tasks.

Message Queue Control Block

A data structure associated with each message queue used by RTEMS
to manage that message queue.

minor number

A numeric value passed to a device
driver, the exact usage of which is driver dependent.

mode

An entry in a task's control block that is
used to determine if the task allows preemption, timeslicing,
processing of signals, and the interrupt disable level used by
the task.

MPCI

An acronym for Multiprocessor Communications
Interface Layer.

multiprocessing

The simultaneous execution of two
or more processes by a multiple processor computer system.

multiprocessor

A computer with multiple CPUs
available for executing applications.

Multiprocessor Communications Interface Layer

A set
of user-provided routines which enable the nodes in a
multiprocessor system to communicate with one another.

Multiprocessor Configuration Table

The data structure defining the characteristics of the multiprocessor
target system with which RTEMS will communicate.

multitasking

The alternation of execution amongst a
group of processes on a single CPU. A scheduling algorithm is
used to determine which process executes at which time.

mutual exclusion

A term used to describe the act of
preventing other tasks from accessing a resource simultaneously.

nested

A term used to describe an ASR that occurs
during another ASR or an ISR that occurs during another ISR.

node

A term used to reference a processor running
RTEMS in a multiprocessor system.

non-existent

The state occupied by an uncreated or
deleted task.

numeric coprocessor

A component used in computer
systems to enhance performance in mathematically intensive
situations. It is typically viewed as a logical extension of
the primary processor.

object

In this document, this term is used to refer
collectively to tasks, timers, message queues, partitions,
regions, semaphores, ports, and rate monotonic periods. All
RTEMS objects have IDs and user-assigned names.

object-oriented

A term used to describe systems
with common mechanisms for utilizing a variety of entities.
Object-oriented systems shield the application from
implementation details.

operating system

The software which controls all
the computer's resources and provides the base upon which
application programs can be written.

overhead

The portion of the CPUs processing power
consumed by the operating system.

packet

A buffer which contains the messages passed
between nodes in a multiprocessor system. A packet is the
contents of an envelope.

partition

An RTEMS object which is used to allocate
and deallocate fixed size blocks of memory from an dynamically
specified area of memory.

Partition Control Block

A data structure associated
with each partition used by RTEMS to manage that partition.

pending

A term used to describe a task blocked
waiting for an event, message, semaphore, or signal.

periodic task

A task which must execute at regular
intervals and comply with a hard deadline.

physical address

The actual hardware address of a
resource.

poll

A mechanism used to determine if an event has
occurred by periodically checking for a particular status.
Typical events include arrival of data, completion of an action,
and errors.

pool

A collection from which resources are
allocated.

portability

A term used to describe the ease with
which software can be rehosted on another computer.

posting

The act of sending an event, message,
semaphore, or signal to a task.

preempt

The act of forcing a task to relinquish the
processor and dispatching to another task.

priority

A mechanism used to represent the relative
importance of an element in a set of items. RTEMS uses priority
to determine which task should execute.

priority inheritance

An algorithm that calls for
the lower priority task holding a resource to have its priority
increased to that of the highest priority task blocked waiting
for that resource. This avoids the problem of priority
inversion.

priority inversion

A form of indefinite
postponement which occurs when a high priority tasks requests
access to shared resource currently allocated to low priority
task. The high priority task must block until the low priority
task releases the resource.

processor utilization

The percentage of processor
time used by a task or a set of tasks.

proxy

An RTEMS control structure used to represent,
on a remote node, a task which must block as part of a remote
operation.

Proxy Control Block

A data structure associated
with each proxy used by RTEMS to manage that proxy.

PTCB

An acronym for Partition Control Block.

PXCB

An acronym for Proxy Control Block.

quantum

The application defined unit of time in
which the processor is allocated.

queue

Alternate term for message queue.

QCB

An acronym for Message Queue Control Block.

ready

A task occupies this state when it is
available to be given control of the CPU.

real-time

A term used to describe systems which are
characterized by requiring deterministic response times to
external stimuli. The external stimuli require that the
response occur at a precise time or the response is incorrect.

reentrant

A term used to describe routines which do
not modify themselves or global variables.

region

An RTEMS object which is used to allocate
and deallocate variable size blocks of memory from a dynamically
specified area of memory.

Region Control Block

A data structure associated
with each region used by RTEMS to manage that region.

registers

Registers are locations physically
located within a component, typically used for device control or
general purpose storage.

remote

Any object that does not reside on the local
node.

remote operation

The manipulation of an object
which does not reside on the same node as the calling task.

return code

Also known as error code or return
value.

resource

A hardware or software entity to which
access must be controlled.

resume

Removing a task from the suspend state. If
the task's state is ready following a call to the
rtems_task_resume
directive, then the task is available for scheduling.

return code

A value returned by RTEMS directives to
indicate the completion status of the directive.

RNCB

An acronym for Region Control Block.

round-robin

A task scheduling discipline in which
tasks of equal priority are executed in the order in which they
are made ready.

RS-232

A standard for serial communications.

running

The state of a rate monotonic timer while
it is being used to delineate a period. The timer exits this
state by either expiring or being canceled.

schedule

The process of choosing which task should
next enter the executing state.

schedulable

A set of tasks which can be guaranteed
to meet their deadlines based upon a specific scheduling
algorithm.

segments

Variable sized memory blocks allocated
from a region.

semaphore

An RTEMS object which is used to
synchronize tasks and provide mutually exclusive access to
resources.

Semaphore Control Block

A data structure associated
with each semaphore used by RTEMS to manage that semaphore.

shared memory

Memory which is accessible by
multiple nodes in a multiprocessor system.

signal

An RTEMS provided mechanism to communicate
asynchronously with a task. Upon reception of a signal, the ASR
of the receiving task will be invoked.

signal set

A thirty-two bit entity which is used to
represent a task's collection of pending signals and the signals
sent to a task.

SMCB

An acronym for Semaphore Control Block.

soft real-time system

A real-time system in which a
missed deadline does not compromise the integrity of the system.

sporadic task

A task which executes at irregular
intervals and must comply with a hard deadline. A minimum
period of time between successive iterations of the task can be
guaranteed.

stack

A data structure that is managed using a Last
In First Out (LIFO) discipline. Each task has a stack
associated with it which is used to store return information
and local variables.

status code

Also known as error code or return
value.

suspend

A term used to describe a task that is not
competing for the CPU because it has had a
rtems_task_suspend directive.

synchronous

Related in order or timing to other
occurrences in the system.

system call

In this document, this is used as an
alternate term for directive.

target

The system on which the application will
ultimately execute.

task

A logically complete thread of execution. The
CPU is allocated among the ready tasks.

Task Control Block

A data structure associated with
each task used by RTEMS to manage that task.

task switch

Alternate terminology for context
switch. Taking control of the processor from one task and given
to another.

TCB

An acronym for Task Control Block.

tick

The basic unit of time used by RTEMS. It is a
user-configurable number of microseconds. The current tick
expires when the rtems_clock_tick
directive is invoked.

tightly-coupled

A multiprocessor configuration
system which communicates via shared memory.

timeout

An argument provided to a number of
directives which determines the maximum length of time an
application task is willing to wait to acquire the resource if
it is not immediately available.

timer

An RTEMS object used to invoke subprograms at
a later time.

Timer Control Block

A data structure associated
with each timer used by RTEMS to manage that timer.

timeslicing

A task scheduling discipline in which
tasks of equal priority are executed for a specific period of
time before being preempted by another task.

timeslice

The application defined unit of time in
which the processor is allocated.

TMCB

An acronym for Timer Control Block.

transient overload

A temporary rise in system
activity which may cause deadlines to be missed. Rate Monotonic
Scheduling can be used to determine if all deadlines will be met
under transient overload.

user extensions

Software routines provided by the
application to enhance the functionality of RTEMS.

User Extension Table

A table which contains the
entry points for each user extensions.

User Initialization Tasks Table

A table which
contains the information needed to create and start each of the
user initialization tasks.

user-provided

Alternate term for user-supplied.
This term is used to designate any software routines which must
be written by the application designer.

user-supplied

Alternate term for user-provided.
This term is used to designate any software routines which must
be written by the application designer.

vector

Memory pointers used by the processor to
fetch the address of routines which will handle various
exceptions and interrupts.

wait queue

The list of tasks blocked pending the
release of a particular resource. Message queues, regions, and
semaphores have a wait queue associated with them.